BY MARTIN VAUGHAN AND SAM HOLMES SINGAPORE—Singapore's central bank expanded a two-month old probe into how banks set benchmark rates, sweeping industry-set reference rates for certain Asian currencies into its investigation. The announcement opens a potential new front in a global inquiry by regulators into suspected attempts by banks to influence rates that until now had focused on interest-rate benchmarks like the London Interbank Offered Rate, or Libor. The Monetary Authority of Singapore said it has instructed banks to review their processes to set foreign-exchange rates used to settle certain currency forward contracts.